Many packaging applications use resealable containers to store various types of articles and materials. These packages may be used to store and ship food products, non-food consumer goods, printed matter, correspondence, medical supplies, waste materials, and many other articles.
Resealable packages are convenient in that they can be closed and resealed after the initial opening to preserve the enclosed contents. The need to locate a storage container for the unused portion of the products in the package is thus avoided. As such, providing products in resealable packages appreciably enhances the marketability of those products.
A common method of packaging foods, such as sliced luncheon meats and the like, is by use of vertical or horizontal form, fill, and seal procedures. These procedures involve shaping a portion of film ("form"), placing the food article inside or upon the formed film portion ("fill"), completing the closure of the film portion around the food article and "sealing" open edges to complete the packaging process.
The "sealing" stage often involves using a resealable closure mechanism. The resealable closure mechanism is often produced as a separate item from the package and is attached to and made integral with the package at a later point in the manufacturing process by a heat and pressure sealing process. Each separate closure profile includes a base strip and an interlocking member. In some conventional implementations, the closure profiles are formed by two separate extrusions or through two separate openings of a common extrusion die. One closure profile can have a rib or male member and the other a mating groove or female member. The male or female member extends from the front face of the base strip. The rib and groove form a pressure-fastenable and releasable closure mechanism. The back side, or sometimes an extended portion of the front face of the base strip, is sealed to the package film so that the closure mechanism is disposed between the package walls adjacent to the openable side of the package. In some implementations, the packages are made of polymeric materials, since these materials inhibit the migration of air and water from and into the package. The packages can be either flexible or rigid.
Attaching a resealable closure mechanism to the package is fairly straightforward when both the mechanism and the package material are made of compatible materials. Applying heat and pressure in the typical heat-sealing process fuses the mechanism and package together. For example, a resealable polyethylene sandwich bag having a polyethylene closure mechanism can be manufactured in this manner. Alternatively, in some arrangements, the packages and closure mechanisms are made from different materials. For example, packages made from polyester not only protect food from minor air and water losses, but also minimize flavor scalping from the food. However, these polyester packages often have polyethylene or polypropylene closure mechanisms. In such mechanisms, an attachment layer between the closure mechanism and the package is used that adequately bonds to both of the dissimilar materials. Thus, one side of the attachment layer bonds with the material of the closure mechanism, while the other side of the attachment layer bonds with the material of the package. Generally, the attachment layer is coextruded with the closure mechanism so as to bond the attachment layer to the closure mechanism. The difference in the coefficients of thermal expansion render it difficult to coextrude a complete sheet of attachment material with the closure mechanism without causing the closure profile to become distorted after the two pieces are actively or passively cooled.
When attachment layers are used, particular attention is placed on the heat-sealing process. To ensure an adequate bond, the dwell time and temperature of the heating process should cause both sides of the attachment layer to melt and bond with their corresponding parts. Additionally, if the heat-sealing bar applying the heat load into the attachment layer becomes misaligned, the attachment layer might not fully adhere to the closure mechanism or the package. If the heat-sealing process is not perfonned adequately, the package may leak under the closure mechanism. This defect can enlarge when the consumer separates the male and female closure members to open the closure mechanism. Thus, the bond should be strong enough to resist tearing at this interface.
Another problem encountered when heat-sealing is applied to the attachment layer is heat-related distortion of the underlying profile layer. If the underlying profile layer is severely distorted, the package will not provide a hermetic seal. Furthermore, the profile layer may even melt, preventing the profile from functioning properly.